The present invention relates to a wheel drive mode changeover apparatus for an agricultural tractor, and more particularly to an apparatus for permitting changeover between a two wheel drive mode and a four wheel drive mode to be effected according to traveling conditions of the tractor such as a sudden sharp turn, a small turn and bad road conditions.
The above apparatus provided for a four wheel drive vehicle is well known, which permits the change-over between the two wheel drive mode and the four wheel drive mode to be effected manually or automatically according to road conditions. One such example is disclosed in U.S. Pat. No. 4,484,653.
An apparatus for use in a four wheel drive vehicle for permitting changeover between a front wheel accelerating four wheel drive mode having front wheels driven at a higher peripheral speed than rear wheels and a standard four wheel drive mode having the front and rear wheels driven at a substantially equal peripheral speed is known, for example, from a Japanese patent application laid open under No. 176121/84 (the corresponding UK application being published under No. GB 2136784A) and from U.S. Pat. No. 4,574,910. In this type of wheel drive mode changeover apparatus, the wheel drive mode changeover is effected in response to detection of a steering angle or of braking of a right rear wheel or left rear wheel. The traveling conditions and the wheel drive modes are in one-to-one relations where, as in the above construction, the wheel drive modes are changed in response to detection of an amount of displacement of a control device for varying the traveling conditions of the vehicle, such as a steering wheel or a side brake pedal, that is to say where the wheel drive modes are changed according to changes in the traveling conditions of the vehicle. Therefore, a desired wheel drive mode cannot be selected under cetain traveling conditions at will, and a certain wheel drive mode cannot be retained against changes in the traveling conditions.
The four wheel drive mode having the front wheels accelerated is used when a small turn is necessary or when making a sudden sharp turn to avert a danger just ahead. For a normal agricultural operation or traveling the four wheel drive mode having the front and rear wheels driven at a substantially equal peripheral speed or the two wheel drive mode is used. Therefore, to establish the one-to-one relations between these drive modes and the traveling conditions does not provide for but rather deviates from the optimal use of these three drive modes.